A Rare Case of Metaplastic Thymoma Presenting With Myasthenia Gravis.

Thymomas are tumors of the mediastinum often associated with autoimmune conditions, in particular myasthenia gravis. In contrast, among the fewer than 40 reports of metaplastic thymoma, myasthenia gravis is rarely found. We describe the fourth patient, and first man, with metaplastic thymoma and myasthenia gravis. A 34-year-old had acute onset of double vision with associated dysphagia and was found to have an elevation of serum acetylcholine receptor antibodies. He underwent a transsternal thymectomy. Tissue sections showed a biphasic proliferation of keratin-positive epithelial cells with a complement of spindle cells confirming the diagnosis of metaplastic thymoma. Terminal deoxynucleotidyl transferase (TDT)-positive T lymphocytes were rare and only found in the periphery of the tumor, consistent with thymic remnant. A YAP1::MAML2 gene fusion, with an in-frame fusion between genes YAP1 Exon5 (NM_001130145) and MAML2 Exon2 (NM_032427) was found, supporting further the diagnosis of metaplastic thymoma (Anchored multiplex RNA sequencing [Archer Dx, Boulder, CO] assay). The patient's gender and relatively young age, the presence of an autoimmune condition, and the lack of lymphocytic infiltrate all contribute unusual features to this case and suggest avenues for further exploration.
Competing Interests: Declaration of Conflicting InterestsThe author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Henry J. Kaminski, M.D., is a consultant for Roche, Cabeletta Bio, and UCB Pharmaceuticals; and is CEO and CMO of ARC Biotechnology, LLC based on US Patent 8,961,98. He is principal investigator of the Rare Disease Network for Myasthenia Gravis (MGNet) National Institute of Neurological Disorders & Stroke, U54 NS115054, Targeted Therapy for Myasthenia Gravis. R41 NS110331 to ARC Biotechnology, and co-investigator for R43NS124329 MV2C2 antibody as a new therapeutic for myasthenia gravis to Mimivax, LLC. Keith Mortman, M.D., is a consultant for Ethicon & on the advisory board for Guardant Health. We have no other potential conflicts to disclose.